Laser printing directs beams of laser light to a photo-conducting drum in order to electro-statically charge the surface of the drum. The laser illuminated drum regions electrostatically attracts toner particles which are subsequently transferred to a piece of paper using mechanical pressure and heat. Thus, the laser illuminated drum regions generally correspond to the printed matter on the paper.
Laser printers print images by scanning a laser beam using a polygonal mirror that rotates at high speed. The printing speed may be determined, in part, by the laser beam scanning speed, which depends on the rotational speed of the polygonal mirror. However, along with faster printing speeds, the demanded rotational speed of the motor that rotates the polygonal mirror is also increasing year by year, but the rotational speed of the motor is starting to hit the point of diminishing returns. Therefore, other technologies are being developed to achieve even higher printing speeds.
As laser printing speeds increase and print resolution becomes higher, faster laser beam scanning speeds are being demanded. Multi-beam laser diode components can increase the effective scanning speed by scanning multiple lines onto the drum surface in a single pass. Current technology employs anywhere from four to twelve laser beams or more per print head.
Multi-beam laser diodes emit multiple laser beams from a single semiconductor device. By using quad-beam or twelve-beam laser diodes, the printing speed can theoretically be increased up to 4 times or 12 times (or higher) as compared to previous scanning speeds.
FIG. 1 is a diagram illustrating the operation of a multi-beam laser diode system. The diagram illustrates a laser array that is used to expose the photoconductor drum. The illustrated laser array contains 12 emitters.
The beam from each emitter is moved across the page to expose the rows of the image. The beam is switched on when a dot is desired to be developed on the page. A set of twelve rows from the image (called a swath) is exposed simultaneously. When the beam reaches the side of the page, it returns to the other side to start scanning again. The photoconductor has advanced, so the next twelve rows (or swath two) will be exposed.
In the laser printing or image forming systems that employ multi-beam systems, it is beneficial to control the write timings of each of the light beams used to write the images on the photoconductive drum or body. In other words, it is preferred if the write start positions, write timings, and spacing of each of the light beams on the photoconductive body or drum accurately match.
Sometimes a beam detector is provided outside an effective scan region of the plurality of light beams, and one (or more) of the plurality of light beams is controlled so that this selected light beam passes the beam detector in an “on” state. Electrical modulating signals are generated to modulate the plurality of light beams, based on an output of the beam detector. The modulating signals are delayed and controlled depending on the arrangements of the plurality of light beams, so that positions and timing of the plurality of light beams match on the recording medium.
The other parameter which is desired to be controlled is the vertical distance between laser emitters, which will determine the accuracy of the vertical position of the printed dot. In general, each light emitting position of the semiconductor laser array may be positioned with relative accuracy during the production process of the beam recording apparatus. However, due to inconsistencies introduced by processing errors, optical magnification errors, and assembling errors of components (e.g., the light source, photoconductive body, etc.) slight errors may introduced into the optical magnification from the light source to the photoconductive body. Errors in drum rotation speed can also exist. These errors may be unique to each machine and are generally unpredictable before assembly of a model is complete. These errors can make it difficult to accurately provide the highly accurate output that is desired in high quality imaging and printing devices.